[squid-users] Bypassing access entries by means of url_regex

With this setup

--
acl ur_foo url_regex www.foobar.org
         
http_access allow ur_foo
http_access deny all 
--
my users can bypass the access entries requesting
www.notallowed.com/?www.foobar.org
the clear solution for me is to force the string www.foobar.org to be at
the beggining of the request
acl ur_foo url_regex ^www.foobar.org
but that way I can't access www.foobar.org.
Do exists a workaround for this?
Regards,
maykel